Siemens SIMATIC PCS 7 System Manual page 85

A configured substitute value of 0 mA will produce at least these output values. In a
redundant configuration of analog outputs, the substitute value of the current outputs is
automatically set permanently to "zero current and zero voltage". You can also specify a
configurable compensation current of 0-400 µA for an output range of 4-20 mA.
This means you have the option of matching the minimum/compensation current to the
connected I/O.
To minimize the error of the total current at the summing point in case of one-sided
passivation, the assigned compensation current is subtracted in this case from the current of
the depassivated (i.e., active) channel with a pre-set value of 4 mA (range +-20 µA).
Note
If both channels of a channel pair were passivated (e.g., by OB 85), the respective half of the
current value is still output to both storage locations in the process image of outputs. If one
channel is depassivated, then the full value is output on the available channel. If this is not
required, a substitute value must be written to the lower channels of both modules prior to
executing FB 451 "RED_OUT".
Depassivation of modules
Passivated modules are depassivated by the following events:
● When the fault-tolerant system starts up
● When the fault-tolerant system changes over to "redundant" operating state
● After system modifications during operation
● If you call FC 451 "RED_DEPA" and at least one redundant channel or module is
passivated.
The depassivation is executed in FB 450 "RED IN" after one of these events has occurred.
Completion of the depassivation of all modules is logged in the diagnostics buffer.
Note
When a redundant module is assigned a process image partition and the corresponding OB
is not available on the CPU, the complete passivation process may take approximately 1
minute.
See also
S7-400H Systems Redundant I/O
(http://support.automation.siemens.com/WW/view/en/9275191)
CPU 410-5H Process Automation/CPU 410 SMART
System Manual, 09/2014, A5E32631667-AB
4.7 Connecting redundant I/O to the PROFIBUS DP interface
I/O configuration variants
85